1. Field of the Invention
The present invention relates generally to a fluid-filled type vibration damping device used for an automotive engine mount or the like. More particularly, the present invention pertains to a fluid-filled type vibration damping device capable of achieving vibration damping effect based on flow action of a fluid with respect to any of two or more vibration inputs having different frequencies.
2. Description of the Related Art
Conventionally, there is known a vibration damping device interposed between components that make up a vibration transmission system so as to elastically connect or elastically support those components. A fluid-filled type vibration damping device, which is one type of the vibration damping device, is adapted for use as an automotive engine mount or the like. The fluid-filled type vibration damping device includes a first mounting member, a second mounting member, a main rubber elastic body elastically connecting the first and second mounting members, a partition member supported by the second mounting member, and a pressure-receiving chamber and an equilibrium chamber disposed on either side of the partition member. The pressure-receiving chamber whose wall is partially defined by the main rubber elastic body is adapted to give rise to internal pressure fluctuations, while the equilibrium chamber whose wall is partially defined by a flexible film is adapted to permit changes in volume. A non-compressible fluid fills each of the chambers. In addition, the pressure-receiving chamber and the equilibrium chamber are interconnected through a first orifice passage and a second orifice passage, with the second orifice passage tuned to a higher frequency than the first orifice passage. At times of vibration input, a fluid flow will be produced between the pressure-receiving chamber and the equilibrium chamber so as to exhibit vibration damping effect based on resonance action or other flow action of the fluid. Japanese Unexamined Patent Publication No. JP-A-2007-155033 discloses one example of such a fluid-filled type vibration damping device, in which the second orifice passage is constituted by upper and lower through holes and a housing space of a movable rubber plate.
The fluid-filled type vibration damping device incorporating the first orifice passage and the second orifice passage having different tuning frequencies is sometimes furnished with a switching mechanism. This switching mechanism switches the second orifice passage, which is tuned to the higher frequency, between open state and closed state in order to effectively exhibit vibration damping effect of both orifice passages. Specifically, JP-A-2007-155033 discloses that the movable rubber plate is disposed on the path of the second orifice passage. At times of input of low-frequency, large-amplitude vibration, the movable rubber plate is pressed against the partition member while blocking off the upper and lower through holes, thereby closing the second orifice passage. This will ensure a sufficient amount of fluid flow through the first orifice passage.
However, with the fluid-filled type vibration damping device disclosed in JP-A-2007-155033, an impact during abutment of the movable rubber plate against the partition member may be transmitted as a noise to the vehicle body via the second mounting member. In particular, with the switching mechanism by means of the movable plate, the direction of exertion of pressure on the movable plate is generally coincident with the direction of abutment of the movable plate against the partition member. Accordingly, a differential in fluid pressure between the pressure-receiving chamber and the equilibrium chamber is likely to exert on the movable plate as an accelerating force, so that possible striking noise during abutment between the movable plate and the partition member tends to be a problem. Moreover, the movable plate is not supported by the partition member and freely displaces in the housing space without being appreciably decelerated. This makes it difficult to reduce the impact during the abutment.